Can't it also be 'what is outside?'?
I asked an Indonesian and it can’t. Apparently this construction means “what is going on”
Depending on the context, yes it can. For example: I am looking outside the window and my kid asks "ada apa diluar" and I say "ada kucing di luar". Jadi in this case "ada apa diluar" translates to "what is outside".
What if "Apa ada di luar"?
Would apa yang terjadi di luar be acceptable??
Apa yang berlaku di luar is more to what is going on outside there.
There are 2 ways this is be handled, the spoken by folks outside the street which is made easy and the proper offical - this is spoken street language